1. Field of the Invention
The present invention relates to a power output device comprising an internal combustion engine and at least an electric motor connected to the engine mechanically, and more particularly a method of controlling the power output device, and a hybrid vehicle having the power output device.
2. Description of Related Art
In recent years a hybrid vehicle having an internal combustion engine and at least one electric motor has been proposed. In such a hybrid vehicle, several kinds of structures were proposed. One of them is called a parallel hybrid vehicle. In such a parallel hybrid vehicle, mechanical power transmitted from the internal combustion engine and/or electric power generated by the electric motor can be transmitted to a wheel shaft of the hybrid vehicle.
The hybrid vehicle can be driven, even when the engine is sometimes not working or in an idling-condition. When the engine stops or is in an idling-condition and the hybrid vehicle is being driven, the torque outputted from the engine is substantially zero. If a speed of the vehicle and a target rotation speed of the engine is given, a target rotation speed of the electric motor is determined. The necessary torque for keeping the rotation speed of the electric motor in the target rotation speed is outputted by the proportional integration control. The present rotation speed of the electric motor is detected by a sensor. If the rotation speed is less than the target rotation speed, a positive torque of the electric motor is provided in order to increase the rotation speed of the electric motor. On the contrary, if it is more than the target rotation speed, a load is added to the electric motor in order to reduce the rotation speed. On the other hand, a throttle opening angle and/or fuel injection to the engine are controlled so that a predetermined idling rotation speed of the engine is maintained.
The rotation speed of the electric motor sensed by the sensor has a detected error or a fluctuation caused by backlashes of gears installed in a power train of this power output device, or vibration of the vehicle. Furthermore the rotation speed of the engine fluctuates. Consequently, during the above-mentioned control, an electric current is fed substantially consecutively for correcting the fluctuation of the rotation speed of the electric motor. The electric motor consumes electric power on one occasion, and on another occasion regenerates by the aforementioned control. If the electric power continues to be consumed when the engine does not output power substantially, a battery storage may run out of electricity. On the contrary if the regeneration of the electric motor continues, the battery may be over-charged. Furthermore even during the stopping condition of the engine, such a phenomena above mentioned could occur by controlling the electric motor in response to vibrations of the hybrid vehicle. Furthermore in special cases the electric motor could rotate the engine, though the engine needs not rotate.
The rotation speed detected by the sensor used in the control of the electric motor may happen to be inconsistent with the rotation speed detected by a sensor in control of the engine. This inconsistency is caused by the discrepancy of the characteristics or the detecting cycles between the two above-mentioned sensors. By this inconsistency, the electric motor could continue to consume or to regenerate electric power. For example, when the rotation speed of the electric motor is higher than the target rotation speed, the electric motor is controlled to reduce the rotation speed through the regeneration of the electric motor. If the detected rotation speed of the engine is lower than the target rotation speed when the rotation speed of the electric motor converges to the target rotation speed, the engine is controlled to increase the rotation speed. Then the rotation speed of the electric motor is higher than the target rotation speed and the electric motor begins to regenerate again. By these reiterations of the control of the electric motor and the engine, the electric motor continues the regenerating operation. This phenomenon is one of the problems relating to a mutual intervention between the control of the electric motor and the engine.
Furthermore, another problem caused by the mutual intervention occurs as follows. The operating condition of the electric motor can not sufficiently follow a fluctuation of the rotation speed of the engine, because a control usually entails a time delay. Such a time delay causes a fluctuation of the rotation speed of the engine. When the engine is in an idling-condition, the rotation speed of the engine is controlled to be at a predetermined idling rotation speed. This control has, of course, a time delay. Consequently the operating condition of the engine happens to be very unstable by the mutual effects of the time delays of both controls. When a positive torque is outputted to the electric motor and results in the rotation speed of the engine being higher than the idling rotation speed, the amount of fuel injected into the engine could be expected to be reduced and the rotation speed of the engine could be expected to converge with the predetermined idling rotation speed. The rotation speed of the engine, however, could happen to be lower than the idling rotation speed, because the torque of the electric motor lowers.
When the engine does not output torque substantially and the electric motor is feedback-controlled, the above-mentioned problems occur in the conventional hybrid vehicle which have an internal combustion engine, an electric motor, and a drive shaft mechanically connected together.